Catalyzed SCR Filter and Emission Treatment System

ABSTRACT

Provided is an emission treatment system and method for simultaneously remediating the nitrogen oxides (NOx), particulate matter, and gaseous hydrocarbons present in diesel engine exhaust streams. The emission treatment system has an oxidation catalyst upstream of a soot filter coated with a material effective in the Selective Catalytic Reduction (SCR) of NOx by a reductant, e.g., ammonia. Also provided is a method for disposing an SCR catalyst composition on a wall flow monolith that provides adequate catalyst loading, but does not result in unsuitable back pressures in the exhaust.

This application is a continuation of U.S. application Ser. No.11/676,798, filed Feb. 20, 2007, which is a divisional application ofU.S. application Ser. No. 10/634,659, filed Aug. 5, 2003, the content ofboth applications being hereby incorporated by reference in theirentirety.

The present invention relates to an emission treatment system having anoxidation catalyst upstream of a soot filter coated with a materialeffective in the Selective Catalytic Reduction (SCR) of NOx by areductant, e.g., ammonia. In one embodiment, the system provides aneffective method of simultaneously remediating the nitrogen oxides(NOx), particulate matter, and gaseous hydrocarbons present in dieselengine exhaust streams.

Diesel engine exhaust is a heterogeneous mixture which contains not onlygaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons(“HC”) and nitrogen oxides (“NO_(x)”), but also condensed phasematerials (liquids and solids) which constitute the so-calledparticulates or particulate matter. Often, catalyst compositions andsubstrates on which the compositions are disposed are provided in dieselengine exhaust systems to convert certain or all of these exhaustcomponents to innocuous components. For example, diesel exhaust systemscan contain one or more of a diesel oxidation catalyst, a soot filterand a catalyst for the reduction of NOx.

Oxidation catalysts that contain platinum group metals, base metals andcombinations thereof are known to facilitate the treatment of dieselengine exhaust by promoting the conversion of both HC and CO gaseouspollutants and some proportion of the particulate matter throughoxidation of these pollutants to carbon dioxide and water. Suchcatalysts have generally been contained in units called diesel oxidationcatalysts (DOC's), which are placed in the exhaust of diesel engines totreat the exhaust before it vents to the atmosphere. In addition to theconversions of gaseous HC, CO and particulate matter, oxidationcatalysts that contain platinum group metals (which are typicallydispersed on a refractory oxide support) also promote the oxidation ofnitric oxide (NO) to NO₂.

The total particulate matter emissions of diesel exhaust are comprisedof three main components. One component is the solid, dry, solidcarbonaceous fraction or soot fraction. This dry carbonaceous mattercontributes to the visible soot emissions commonly associated withdiesel exhaust. A second component of the particulate matter is thesoluble organic fraction (“SOF”). The soluble organic fraction issometimes referred to as the volatile organic fraction (“VOF”), whichterminology will be used herein. The VOF can exist in diesel exhausteither as a vapor or as an aerosol (fine droplets of liquid condensate)depending on the temperature of the diesel exhaust. It is generallypresent as condensed liquids at the standard particulate collectiontemperature of 52° C. in diluted exhaust, as prescribed by a standardmeasurement test, such as the U.S. Heavy Duty Transient Federal TestProcedure. These liquids arise from two sources: (1) lubricating oilswept from the cylinder walls of the engine each time the pistons go upand down; and (2) unburned or partially burned diesel fuel.

The third component of the particulate matter is the so-called sulfatefraction. The sulfate fraction is formed from small quantities of sulfurcomponents present in the diesel fuel. Small proportions of SO₃ areformed during combustion of the diesel, which in turn combines rapidlywith water in the exhaust to form sulfuric acid. The sulfuric acidcollects as a condensed phase with the particulates as an aerosol, or isadsorbed onto the other particulate components, and thereby adds to themass of TPM.

One key aftertreatment technology in use for high particulate matterreduction is the diesel particulate filter. There are many known filterstructures that are effective in removing particulate matter from dieselexhaust, such as honeycomb wall flow filters, wound or packed fiberfilters, open cell foams, sintered metal filters, etc. However, ceramicwall flow filters, described below, receive the most attention. Thesefilters are capable of removing over 90% of the particulate materialfrom diesel exhaust. The filter is a physical structure for removingparticles from exhaust, and the accumulating particles will increase theback pressure from the filter on the engine. Thus, the accumulatingparticles have to be continuously or periodically burned out of thefilter to maintain an acceptable back pressure. Unfortunately, thecarbon soot particles require temperatures in excess of 500° C. to burnunder oxygen rich (lean) exhaust conditions. This temperature is higherthan what is typically present in diesel exhaust.

Provisions are generally introduced to lower the soot burningtemperature in order to provide for passive regeneration of the filter.The presence of a catalyst promotes soot combustion, therebyregenerating the filters at temperatures accessible within the dieselengine's exhaust under realistic duty cycles. In this way a catalyzedsoot filter (CSF) or catalyzed diesel particulate filter (CDPF) iseffective in providing for >80% particulate matter reduction along withpassive burning of the accumulating soot, and thereby promoting filterregeneration.

Future emissions standards adopted throughout the world will alsoaddress NOx reductions from diesel exhaust. A proven NOx abatementtechnology applied to stationary sources with lean exhaust conditions isSelective Catalytic Reduction (SCR). In this process, NOx is reducedwith ammonia (NH₃) to nitrogen (N₂) over a catalyst typically composedof base metals. The technology is capable of NOx reduction greater than90%, and thus it represents one of the best approaches for achievingaggressive NOx reduction goals. SCR is under development for mobileapplications, with urea (typically present in an aqueous solution) asthe source of ammonia. SCR provides efficient conversions of NOx as longas the exhaust temperature is within the active temperature range of thecatalyst.

While separate substrates each containing catalysts to address discretecomponents of the exhaust can be provided in an exhaust system, use offewer substrates is desirable to reduce the overall size of the system,to ease the assembly of the system, and to reduce the overall cost ofthe system. One approach to achieve this goal is to coat the soot filterwith a catalyst composition effective for the conversion of NOx toinnocuous components. With this approach, the catalyzed soot filterassumes two catalyst functions: removal of the particulate component ofthe exhaust stream and conversion of the NOx component of the exhauststream to N₂.

Coated soot filters that can achieve NOx reduction goals require asufficient loading of SCR catalyst composition on the soot filter. Thegradual loss of the catalytic effectiveness of the compositions thatoccurs over time through exposure to certain deleterious components ofthe exhaust stream augments the need for higher catalyst loadings of theSCR catalyst composition. However, preparation of coated wall flow sootfilters with higher catalyst loadings can lead to unacceptably high backpressure within the exhaust system. Coating techniques that allow highercatalyst loadings on the wall flow filter, yet still allow the filter tomaintain flow characteristics that achieve acceptable back pressures aretherefore desirable.

An additional aspect for consideration in coating the wall flow filteris the selection of the appropriate SCR catalyst composition. First, thecatalyst composition must be durable so that it maintains its SCRcatalytic activity even after prolonged exposure to higher temperaturesthat are characteristic of filter regeneration. For example, combustionof the soot fraction of the particulate matter often leads totemperatures above 700° C. Such temperatures render many commonly usedSCR catalyst compositions such as mixed oxides of vanadium and titaniumless catalytically effective. Second, the SCR catalyst compositionspreferably have a wide enough operating temperature range so that theycan accommodate the variable temperature ranges over which the vehicleoperates. Temperatures below 300° C. are typically encountered, forexample, at conditions of low load, or at startup. The SCR catalystcompositions are preferably capable of catalyzing the reduction of theNOx component of the exhaust to achieve NOx reduction goals, even atlower exhaust temperatures.

The prior art contains descriptions of the use of SCR catalystcompositions, soot filters and combinations thereof for the abatement ofboth the NOx and particulate components of diesel exhaust. Thesereferences are described below.

Japanese Kokai 3-130522, for example, discloses the treatment of dieselexhaust gases characterized by use of an ammonia injector and porousceramic filter having a denitration catalyst within the pores. Thefilter is installed in the wake of the diesel engine exhaust. Theceramic porous filter comprises an upstream fine pore path layer, and adownstream side course ceramic particle layer on which the denitrationcatalyst was supported. The fine layer can support a platinum orpalladium or other hydrocarbon combustion catalyst. The diesel exhaustgas containing unburned carbon flows through the porous ceramic filterand the carbon particles are filtered onto the surface. The gascontaining nitric oxides and the ammonia passes through the denitrationcatalyst containing side of the filter and the nitric oxides are reducedto nitrogen and water. The oxidation catalyst on the upstream sidecauses the particulate component to burn off catalytically.

U.S. Pat. No. 4,912,776 discloses an oxidation catalyst, an SCR catalystdownstream and adjacent to the SCR catalyst, and a reductant sourceintroduced to the exhaust stream between the oxidation catalyst and theSCR catalyst. Providing a higher feed containing a high proportion ofNO₂ to NO to the SCR reactor is said to allow the use of lowertemperatures and higher space velocities than is possible with a feed ofNO. WO 99/39809 discloses a system for treating combustion exhaust gascontaining NOx and particulates that has an oxidation catalyst effectiveto convert at least a portion of the NO in the NOx to NO₂, a particulatetrap, a source of reductant fluid and an SCR catalyst. The particulatetrap is downstream of the oxidation catalyst; the reductant fluid sourceis downstream of the particulate trap; and the SCR catalyst isdownstream of the reductant fluid source. Reductant fluids disclosedinclude ammonia, urea, ammonium carbamate and hydrocarbons (e.g., dieselfuel).

A catalytic wall flow filter for an exhaust system of a combustionengine is described in WO 01/12320. The wall flow filter has channelsthat are in honeycomb arrangement, where some of the channels areblocked at the upstream end and some of the channels that are unblockedat the upstream end are blocked at the downstream end. An oxidationcatalyst is disposed on a gas impermeable zone at an upstream end ofchannels that are blocked at the downstream end. The filter has a gaspermeable filter zone that is downstream of the oxidation catalyst thatis for trapping soot. The oxidation catalyst is described to be capable(when in an exhaust system) of generating NO₂ from NO to combust thetrapped soot continuously at temperatures below 400° C. The oxidationcatalyst preferably includes a platinum group metal. Exhaust streamscontaining NO are initially passed over the oxidation catalyst toconvert NO to NO₂ prior to filtering to remove soot. The exhaust gasthen containing NO₂ is used to combust the soot trapped on the filter.

In some embodiments of the wall flow filter described in WO 01/12320 thedownstream channels of the soot filter contain a catalyst for a NOxabsorber and an SCR catalyst downstream of the NOx absorber. The SCRcatalyst can be a copper-based material, platinum, a mixed oxide ofvanadium and titania or a zeolite, or mixtures of two or more thereof.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to an emission treatment system fortreatment of an exhaust stream that contains NOx and particulate matter.The emission treatment system includes an oxidation catalyst, aninjector that periodically meters ammonia or an ammonia precursor intothe exhaust stream; and a wall flow monolith. The injector is in fluidcommunication with the oxidation catalyst, and is positioned downstreamof the oxidation catalyst. The wall flow monolith contains an SCRcatalyst composition, is in fluid communication with the injector, andis positioned downstream of the injector.

The wall flow monolith has a plurality of longitudinally extendingpassages formed by longitudinally extending walls bounding and definingsaid passages. The passages include inlet passages that have an openinlet end and a closed outlet end, and outlet passages that have aclosed inlet end and an open outlet end. The wall flow monolith containsan SCR catalyst composition that permeates the walls at a concentrationof at least 1.3 g/in³ (and preferably from 1.6 to 2.4 g/in³). The wallflow monolith has a wall porosity of at least 50% with an average poresize of at least 5 microns. Preferably, the SCR catalyst compositionpermeates the walls of the wall flow monolith so that the walls have awall porosity of from 50 to 75% with an average pore size of from 5 to30 microns.

In a preferred embodiment of the emission treatment system, the SCRcatalyst composition contains a zeolite and base metal componentselected from one or more of a copper and iron component. Preferably,the base metal component is a copper component. Preferred zeolites ofthe SCR catalyst composition have a silica to alumina ratio of at leastabout 10. For instance, a beta zeolite can be used in the SCR catalystcomposition.

Among other things, the oxidation catalyst of the system is useful forcombusting substantial portions of the particulate matter, and inparticular, the VOF, entrained in the exhaust. In addition, asubstantial portion of the NO in the NOx component is oxidized to NO₂over the oxidation catalyst. In preferred embodiments, the oxidationcatalyst is disposed on a honeycomb flow through monolith substrate oran open cell foam substrate. Preferably, the oxidation catalyst includesa platinum group metal component, and in particular, a platinumcomponent. In some embodiments, the oxidation catalyst can also containa zeolite component.

In another preferred embodiment of the emission treatment system, thesystem also has a diesel engine which is located upstream of, and influid communication with the oxidation catalyst.

Another aspect of the invention relates to a method for treatingemissions produced in an exhaust stream that contains NOx andparticulate matter. The method includes:

(a) passing the exhaust stream through an oxidation catalyst wherein asubstantial portion of NO is oxidized to NO₂ to provide an NO₂-enrichedexhaust stream;

(b) metering at periodic intervals, ammonia or an ammonia precursor intothe NO₂-enriched exhaust stream; and,

(c) subsequently passing the exhaust stream through a wall flow monolithwherein particulate matter is filtered and a substantial portion of NOxis reduced to N₂.

Here again, the wall flow monolith has a plurality of longitudinallyextending passages formed by longitudinally extending walls bounding anddefining said passages. The passages include inlet passages that have anopen inlet end and a closed outlet end, and outlet passages that have aclosed inlet end and an open outlet end. The wall flow monolith containsan SCR catalyst composition that permeates the walls at a concentrationof at least 1.3 g/in³ (and preferably from 1.6 to 2.4 g/in³). The wallflow monolith has a wall porosity of at least 50% with an average poresize of at least 5 microns. Preferably, the SCR catalyst compositionpermeates the walls of the wall flow monolith so that the walls have awall porosity of from 50 to 75% with an average pore size of from 5 to30 microns.

In another aspect, the invention relates to a method for disposing anSCR catalyst composition on a wall flow monolith. The method includes:

(a) immersing the wall flow monolith in an aqueous slurry comprising theSCR catalyst composition from a first direction to deposit the SCRcatalyst composition on the inlet passages;

(b) removing excess slurry from the inlet passages by forcing acompressed gas stream through the outlet passages and applying a vacuumto the inlet passages;

(c) immersing the wall flow monolith in the aqueous slurry from a seconddirection, opposite the first direction, to deposit the SCR catalystcomposition on the outlet passages;

(d) removing excess slurry from the outlet passages by forcing acompressed gas stream through the inlet passages and applying a vacuumto the outlet passages; and

(e) drying and calcining the coated wall flow monolith.

The wall flow monolith used in the method preferably has a porosity ofat least 50% (e.g., from 50 to 75%) having a mean pore size of at least5 microns (e.g., from 5 to 30 microns).

Preferably, the SCR catalyst composition permeates the walls at aconcentration of at least 1.3 g/in³ (and preferably from 1.6 to 2.4g/in³).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic depictions of two embodiments of theemission treatment system of the invention;

FIG. 2 shows a perspective view of a wall flow filter substrate;

FIG. 3 shows a cutaway view of a section of a wall flow filtersubstrate;

FIG. 4 shows an embodiment of the emission treatment system of theinvention that includes a urea reservoir and injector;

FIG. 5 is a plot of the DTA signal in microvolts as a function oftemperature for two SCR catalyst compositions mixed with a modelparticulate mass (carbon black and lube oil);

FIG. 6 shows the pressure drop as a function of the air flow for severalcoated wall flow filter substrates and an uncoated wall flow filtersubstrate; and

FIG. 7 is a schematic depiction of a laboratory bench system used toevaluate NOx and particulate reduction for an exemplary emissiontreatment system of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an emission treatment system that effectivelyprovides simultaneous treatment of the particulate matter, the NOx andother gaseous components of diesel engine exhaust. The emissiontreatment system uses an integrated soot filter and SCR catalyst tosignificantly minimize the weight and volume required for the emissionssystem. Moreover, due to the choice of catalytic compositionsimplemented in the system, effective pollutant abatement is provided forexhaust streams of varying temperatures. This feature is advantageousfor operating diesel vehicles under varying loads and vehicle speedswhich significantly impact exhaust temperatures emitted from the enginesof such vehicles.

Integration of NOx reduction and particulate removal functions into asingle catalyst article is accomplished using a wall flow substratecoated with an SCR catalyst composition. Applicants have found a methodfor applying an SCR catalyst composition to a wall flow substrate toform a substrate that can be used in an application where highfiltration efficiency is required. For instance, a substrate formed withthis method is suitable for effectively removing particulate matter(e.g., greater than 80%) in the emission treatment system of theinvention. The coating method disclosed herein allows wall flowsubstrates to be loaded with practical levels of SCR catalyst withoutcausing excessive back pressure across the coated article whenimplemented in emission treatment systems.

Achieving practical levels of SCR catalyst composition on the wall flowsubstrate is important for providing sufficient catalytic activity toachieve mandated NOx reduction levels, and for lowering the combustiontemperature of the soot fraction trapped on the filter. Achievingadequate levels of SCR washcoat compositions on the soot filter is alsoimportant to secure adequate durability for the catalyst. Over extendeduse of the emission treatment system, catalysts are invariably exposedto various levels of catalyst poisons that may be derived through breakdown of lubricating oils, or may arise from impurities in the dieselfuel. Examples of such catalyst poisons include phosphorus, zinc, alkaliand alkaline earth elements. Higher levels of catalyst compositions aretherefore typically deposited on catalyst substrates to overcome theinevitable loss of catalytic activity.

One embodiment of the inventive emission treatment system isschematically depicted in FIG. 1A. As can be seen in FIG. 1A, theexhaust containing gaseous pollutants (including unburned hydrocarbons,carbon monoxide and NOx) and particulate matter is conveyed from theengine 15 to an oxidation catalyst 11. In the oxidation catalyst 11,unburned gaseous and non-volatile hydrocarbons (i.e., the VOF) andcarbon monoxide are largely combusted to form carbon dioxide and water.Removal of substantial proportions of the VOF using the oxidationcatalyst, in particular, helps prevent too great a deposition ofparticulate matter on the soot filter 12 (i.e., clogging), which ispositioned downstream in the system. In addition, a substantialproportion of the NO of the NOx component is oxidized to NO₂ in theoxidation catalyst.

Downstream of the oxidation catalyst is a reductant, in this caseammonia, is injected as a spray via a nozzle (not shown) into theexhaust stream. Aqueous urea shown on one line 18 can serve as theammonia precursor which can be mixed with air on another line 19 in amixing station 16. Valve 14 can be used to meter precise amounts ofaqueous urea which are converted in the exhaust stream to ammonia. Theexhaust stream with the added ammonia is conveyed to the soot filter 12which is coated with an SCR catalyst composition. On passing through thesoot filter, the NOx component is converted through the selectivecatalytic reduction of NOx with ammonia to nitrogen. The increasedproportion of NO₂ in the NOx due to the catalytic action of the upstreamoxidation catalyst facilitates the reduction of the NOx as compared toexhaust streams containing smaller proportions of NO₂ in the NOxcomponent.

Depending on the desired level of NOx removal, additional SCR catalystcan be disposed downstream of the soot filter. For example, theadditional SCR catalyst may be disposed on a monolithic, honeycomb flowthrough substrate or ceramic foam substrate downstream of the sootfilter. Even in these embodiments, the use of the coated SCR soot filterstill achieves a reduction in the total volume of catalyst required tomeet NOx reduction goals.

The particulate matter including the soot fraction and the VOF are alsolargely removed (greater than 80%) by the soot filter. The particulatematter deposited on the soot filter is combusted through theregeneration of the filter, which process is also aided by the presenceof the SCR catalyst composition. The temperature at which the sootfraction of the particulate matter combusts is lowered by the presenceof the catalyst composition disposed on the soot filter.

An optional configuration is shown in FIG. 1B where the emissiontreatment system is provided with a slip oxidation catalyst 13downstream of the coated soot filter 12. The slip oxidation catalyst canbe coated, for example, with a composition containing base metals andless than 0.5 wt % of platinum. This provision can be used to oxidizeany excess NH₃ before it is vented to the atmosphere.

Suitable SCR catalyst compositions for use in the system are able toeffectively catalyze the reduction of the NOx component at temperaturesbelow 600 C, so that adequate NOx levels can be treated even underconditions of low load which typically are associated with lower exhausttemperatures. Preferably, the catalyst article is capable of convertingat least 50% of the NOx component to N₂, depending on the amount ofreductant added to the system. In addition, SCR catalyst compositionsfor use in the system are also ideally able to aid in the regenerationof the filter by lowering the temperature at which the soot fraction ofthe particulate matter is combusted. Another desirable attribute for thecomposition is that it possess the ability to catalyze the reaction ofO₂ with any excess NH₃ to N₂ and H₂O, so that NH₃ is not emitted to theatmosphere.

Useful SCR catalyst compositions used in the inventive system also havethermal resistance to temperatures greater than 650° C. Such hightemperatures are often encountered during the regeneration of sootfilters. Additionally, SCR catalyst compositions should resistdegradation upon exposure to sulfur components, which are often presentin diesel exhaust gas compositions.

Suitable SCR catalyst compositions are described, for instance, in U.S.Pat. Nos. 4,961,917 (the '917 patent) and 5,516,497, which are bothhereby incorporated by reference in their entirety. Compositionsdisclosed in the '917 patent include one or both of an iron and a copperpromoter present in a zeolite in an amount of from about 0.1 to 30percent by weight, preferably from about 1 to 5 percent by weight, ofthe total weight of promoter plus zeolite. In addition to their abilityto catalyze the reduction of NOx with NH₃ to N₂, the disclosedcompositions can also promote the oxidation of excess NH₃ with O₂,especially for those compositions having higher promoter concentrations.

Zeolites used in such compositions are resistant to sulfur poisoning,sustain a high level of activity for the SCR process, and are capable ofoxidation of excess ammonia with oxygen. These zeolites have a pore sizelarge enough to permit adequate movement of the reactant molecules NOand NH₃ in to, and the product molecules N₂ and H₂O out of, the poresystem in the presence of sulfur oxide molecules resulting from shortterm sulfur poisoning, and/or sulfate deposits resulting from long termsulfur poisoning. The pore system of suitable size is interconnected inall three crystallographic dimensions. As is well known to the thoseskilled in the zeolite art, the crystalline structure of zeolitesexhibits a complex pore structure having more or less regularlyrecurring connections, intersections and the like. Pores having aparticular characteristic, such as a given dimension diameter orcross-sectional configuration, are said to be one dimensional if thosepores do not intersect with other like pores. If the pores intersectonly within a given plane with other like pores, the pores of thatcharacteristic are said to be interconnected in two (crystallographic)dimensions. If the pores intersect with other like pores lying both inthe same plane and in other planes, such like pores are said to beinterconnected in three dimensions, i.e., to be “three dimensional”. Ithas been found that zeolites which are highly resistant to sulfatepoisoning and provide good activity for both the SCR process and theoxidation of ammonia with oxygen, and which retain good activity evenwhen subject to high temperatures, hydrothermal conditions and sulfatepoisons, are zeolites which have pores which exhibit a pore diameter ofat least about 7 Angstroms and are interconnected in three dimensions.Without wishing to be bound by any specific theory, it is believed thatthe interconnection of pores of at least 7 Angstroms diameter in threedimensions provides for good mobility of sulfate molecules throughoutthe zeolite structure, thereby permitting the sulfate molecules to bereleased from the catalyst to free a large number of the availableadsorbent sites for reactant NOx and NH₃ molecules and reactant NH₃ andO₂ molecules. Any zeolites meeting the foregoing criteria are suitablefor use in the practices of the present invention; specific zeoliteswhich meet these criteria are USY, Beta and ZSM-20. Other zeolites mayalso satisfy the aforementioned criteria.

When deposited on the wall flow monolith substrates, such SCR catalystcompositions are deposited at a concentration of at least 1.3 g/in³ toensure that the desired NOx reduction and particulate removal levels areachieved and to secure adequate durability of the catalyst over extendeduse. In a preferred embodiment, there is at least 1.6 g/in³ of SCRcomposition, and in particular, 1.6 to 2.4 g/in³, disposed on the wallflow monolith.

Wall flow substrates useful for supporting the SCR catalyst compositionshave a plurality of fine, substantially parallel gas flow passagesextending along the longitudinal axis of the substrate. Typically, eachpassage is blocked at one end of the substrate body, with alternatepassages blocked at opposite end-faces. Such monolithic carriers maycontain up to about 700 or more flow passages (or “cells”) per squareinch of cross section, although far fewer may be used. For example, thecarrier may have from about 7 to 600, more usually from about 100 to400, cells per square inch (“cpsi”). The cells can have cross sectionsthat are rectangular, square, circular, oval, triangular, hexagonal, orare of other polygonal shapes. Wall flow substrates typically have awall thickness between 0.002 and 0.1 inches. Preferred wall flowsubstrates have a wall thickness of between 0.002 and 0.015 inches.

FIGS. 2 and 3 illustrate a wall flow filter substrate 30 which has aplurality of passages 52. The passages are tubularly enclosed by theinternal walls 53 of the filter substrate. The substrate has an inletend 54 and an outlet end 56. Alternate passages are plugged at the inletend with inlet plugs 58, and at the outlet end with outlet plugs 60 toform opposing checkerboard patterns at the inlet 54 and outlet 56. A gasstream 62 enters through the unplugged channel inlet 64, is stopped byoutlet plug 60 and diffuses through channel walls 53 (which are porous)to the outlet side 66. The gas cannot pass back to the inlet side ofwalls because of inlet plugs 58.

Preferred wall flow filter substrates are composed of ceramic-likematerials such as cordierite, α-alumina, silicon carbide, siliconnitride, zirconia, mullite, spodumene, alumina-silica-magnesia orzirconium silicate, or of porous, refractory metal. Wall flow substratesmay also be formed of ceramic fiber composite materials. Preferred wallflow substrates are formed from cordierite and silicon carbide. Suchmaterials are able to withstand the environment, particularly hightemperatures, encountered in treating the exhaust streams.

Preferred wall flow substrates for use in the inventive system includethin porous walled honeycombs (monolith)s through which the fluid streampasses without causing too great an increase in back pressure orpressure across the article. Normally, the presence of a clean wall flowarticle will create a back pressure of 1 inch water column to 10 psig.Ceramic wall flow substrates used in the system are preferably formed ofa material having a porosity of at least 50% (e.g., from 50 to 75%)having a mean pore size of at least 5 microns (e.g., from 5 to 30microns). More preferably, the substrates have a porosity of at least55% and have a mean pore size of at least 10 microns. When substrateswith these porosities and these mean pore sizes are coated with thetechniques described below, adequate levels of SCR catalyst compositionscan be loaded onto the substrates to achieve excellent NOx conversionefficiency. These substrates are still able to retain adequate exhaustflow characteristics, i.e., acceptable back pressures, despite the SCRcatalyst loading. U.S. Pat. No. 4,329,162 is herein incorporated byreference with respect to the disclosure of suitable wall flowsubstrates.

Typical wall flow filters in commercial use are typically formed withlower wall porosities, e.g., from about 35% to 50%, than the wall flowfilters utilized in the invention. In general, the pore sizedistribution of commercial wall flow filters is typically very broadwith a mean pore size smaller than 17 microns.

The porous wall flow filter used in this invention is catalyzed in thatthe wall of said element has thereon or contained therein one or morecatalytic materials. Catalytic materials may be present on the inletside of the element wall alone, the outlet side alone, both the inletand outlet sides, or the wall itself may consist all, or in part, of thecatalytic material. This invention includes the use of one or morelayers of catalytic materials and combinations of one or more layers ofcatalytic materials on the inlet and/or outlet walls of the element.

To coat the wall flow substrates with the SCR catalyst composition, thesubstrates are immersed vertically in a portion of the catalyst slurrysuch that the top of the substrate is located just above the surface ofthe slurry. In this manner slurry contacts the inlet face of eachhoneycomb wall, but is prevented from contacting the outlet face of eachwall. The sample is left in the slurry for about 30 seconds. Thesubstrate is removed from the slurry, and excess slurry is removed fromthe wall flow substrate first by allowing it to drain from the channels,then by blowing with compressed air (against the direction of slurrypenetration), and then by pulling a vacuum from the direction of slurrypenetration. By using this technique, the catalyst slurry permeates thewalls of the substrate, yet the pores are not occluded to the extentthat undue back pressure will build up in the finished substrate. Asused herein, the term “permeate” when used to describe the dispersion ofthe catalyst slurry on the substrate, means that the catalystcomposition is dispersed throughout the wall of the substrate.

The coated substrates are dried typically at about 100° C. and calcinedat a higher temperature (e.g., 300 to 450° C.). After calcining, thecatalyst loading can be determined through calculation of the coated anduncoated weights of the substrate. As will be apparent to those of skillin the art, the catalyst loading can be modified by altering the solidscontent of the coating slurry. Alternatively, repeated immersions of thesubstrate in the coating slurry can be conducted, followed by removal ofthe excess slurry as described above.

A reductant dosing system is provided upstream of the soot filter anddownstream of the oxidation catalyst to inject a NOx reductant into theexhaust stream. As disclosed in U.S. Pat. No. 4,963,332, NOx upstreamand downstream of the catalytic converter can be sensed, and a pulseddosing valve can be controlled by the upstream and downstream signals.In alternative configurations, the systems disclosed in U.S. Pat. No.5,522,218, where the pulse width of the reductant injector is controlledfrom maps of exhaust gas temperature and engine operating conditionssuch as engine rpm, transmission gear and engine speed. Reference isalso made to the discussion of reductant pulse metering systems in U.S.Pat. No. 6,415,602, the discussion of which is hereby incorporated byreference.

In the embodiment of FIG. 4, an aqueous urea reservoir 22 stores aurea/water solution aboard the vehicle which is pumped through a pump 21including a filter and pressure regulator to a urea injector 16. Ureainjector 16 is a mixing chamber which receives pressure regulated air online 19 which is pulsed by a control valve to urea injector 16. Anatomized urea/water/air solution results which is pulse injected througha nozzle 23 into exhaust pipe 24 upstream of the integrated SCR catalystcoated soot filter 12.

This invention is not limited to the aqueous urea metering arrangementshown in FIG. 4. It is contemplated that a gaseous nitrogen basedreagent will be utilized. For example, a urea or cyanuric acid prillinjector can meter solid pellets of urea to a chamber heated by theexhaust gas to gasify the solid reductant (sublimation temperature rangeof about 300 to 400° C.). Cyanuric acid will gasify to isocyanic acid(HNCO) and urea will gasify to ammonia and HNCO. With either reductant,a hydrolysis catalyst can be provided in the chamber and a slip streamof the exhaust gas metered into the chamber (the exhaust gas containssufficient water vapor) to hydrolyze (temperatures of about 150 to 350°C.) HNCO to produce ammonia.

In addition to urea and cyanuric acid, other nitrogen based reducingreagents or reductants especially suitable for use in the control systemof the present invention includes ammelide, ammeline, ammonium cyanate,biuret, cyanuric acid, ammonium carbamate, melamine, tricyanourea, andmixtures of any number of these. However, the invention in a broadersense is not limited to nitrogen based reductants but can include anyreductant containing hydrocarbons such as distillate fuels includingalcohols, ethers, organo-nitro compounds and the like (e.g., methanol,ethanol, diethyl ether, etc.) and various amines and their salts(especially their carbonates), including guanidine, methyl aminecarbonate, hexamethylamine, etc.

Upstream of the reductant dosage system is an oxidation catalyst (orDOC). The oxidation catalyst can be formed from any composition thatprovides effective combustion of unburned gaseous and non-volatilehydrocarbons (i.e., the VOF) and carbon monoxide. In addition, theoxidation catalyst should be effective to convert a substantialproportion of the NO of the NOx component to NO₂. As used herein, theterm “substantial conversion of NO of the NOx component to NO₂” means atleast 20%, and preferably between 30 and 60%. Catalyst compositionshaving these properties are known in the art, and include platinum groupmetal- and base metal-based compositions. The catalyst compositions canbe coated onto honeycomb flow-through monolith substrates formed ofrefractory metallic or ceramic (e.g., cordierite) materials.Alternatively, oxidation catalysts may be formed on to metallic orceramic foam substrates which are well-known in the art. These oxidationcatalysts, by virtue of the substrate on which they are coated (e.g.,open cell ceramic foam), and/or by virtue of their intrinsic oxidationcatalytic activity provide some level of particulate removal.Preferably, the oxidation catalyst removes some of the particulatematter from the exhaust stream upstream of the wall flow filter, sincethe reduction in the particulate mass on the filter potentially extendsthe time before forced regenerations.

One preferred oxidation catalyst composition that may be used in theemission treatment system contains a platinum group component (e.g.,platinum, palladium or rhodium components) dispersed on a high surfacearea, refractory oxide support (e.g., γ-alumina) which is combined witha zeolite component (preferably a beta zeolite). A preferred platinumgroup metal component is platinum. When the composition is disposed on arefractory oxide substrate, e.g., a flow through honeycomb substrate,the concentration of platinum is typically from about 10 to 120 g/ft³ ofplatinum.

Platinum group metal-based compositions suitable for use in forming theoxidation catalyst are also described in U.S. Pat. No. 5,100,632 (the'632 patent) hereby incorporated by reference. The '632 patent describescompositions that have a mixture of platinum, palladium, rhodium, andruthenium and an alkaline earth metal oxide such as magnesium oxide,calcium oxide, strontium oxide, or barium oxide with an atomic ratiobetween the platinum group metal and the alkaline earth metal of about1:250 to about 1:1, and preferably about 1:60 to about 1:6.

Catalyst compositions suitable for the oxidation catalyst may also beformed using base metals as catalytic agents. For example, U.S. Pat. No.5,491,120 (the disclosure of which is hereby incorporated by reference)discloses oxidation catalyst compositions that include a catalyticmaterial having a BET surface area of at least about 10 m²/g and consistessentially of a bulk second metal oxide which may be one or more oftitania, zirconia, ceria-zirconia, silica, alumina-silica, andα-alumina.

Also useful are the catalyst compositions disclosed in U.S. Pat. No.5,462,907 (the '907 patent, the disclosure of which is herebyincorporated by reference). The '907 patent teaches compositions thatinclude a catalytic material containing ceria and alumina each having asurface area of at least about 10 m²/g, for example, ceria and activatedalumina in a weight ratio of from about 1.5:1 to 1:1.5. Optionally,platinum may be included in the compositions described in the '907patent in amounts effective to promote gas phase oxidation of CO andunburned hydrocarbons but which are limited to preclude excessiveoxidation of SO to SO₂. Alternatively, palladium in any desired amountmay be included in the catalytic material.

The following examples further illustrate the present invention, but ofcourse, should not be construed as in any way limiting its scope.

EXAMPLE 1 Coating of Ceramic Wall Flow Filters

Cordierite ceramic wall flow filter substrates (product name C611, NGKInsulators, Ltd.) having dimensions of 5.66×6 inches, a wall thicknessof 0.012 in, an average pore size of 25 microns and 60% wall porositywere used to prepare catalyst-coated soot filters. 150697.5.07

A catalyst slurry containing 27% by weight solids content was formedfrom copper-exchanged beta zeolite (containing 2 wt. % of copper),additional CuSO₄ (sufficient to provide 9.5 wt. % of copper), and 7 wt.% ZrO₂ and de-ionized water (wt. % based on the weight of the betazeolite). The copper-exchanged beta zeolite was prepared as in U.S. Pat.No. 5,516,497.

An identical procedure was used to prepare two of the filter substratesaccording to a preferred embodiment of the invention. The wall flowsubstrate was:

(1) dipped into the slurry to a depth sufficient to coat the channels ofthe substrate along the entire axial length of the substrate from onedirection;

(2) air-knifed the substrate from the side opposite the coatingdirection (i.e., the dry side);

(3) vacuumed from the coated side;

(4) dried at 93° C. for 1 h in flowing air, and calcined at 400° C. for1 h; and

(5) Steps (1) through (4) were then repeated from the opposite side.

These filter substrates (designated as Catalysts A1 and A2) contained acatalyst loading of 2.1 g/in³. The amount of copper contained on thesecatalysts was approximately 0.2 g/in³

Another filter substrate, designated as Catalyst B1, was prepared bycoating a single side of the substrate only, following steps (1) through(4). To reach the same catalyst loading as Catalyst A1, the slurrysolids content was increased to 38%. The composition of the catalystremained the same. Catalyst B1 had a catalyst loading of 2.0 g/in³. Theamount of copper contained on this catalyst was also about 0.2 g/in³.

A reference sample, Catalyst D1, was prepared as a flow through typecatalyst. To prepare a catalyst of this type, a filter substrate of thetype described above was cut across its diameter at one end, just belowthe depth of the plugs. Thus, the wall flow filter was converted into aflow through substrate with effectively half the frontal area blocked.This substrate was coated to obtain a catalyst loading of 2.0 g/in³ ofthe copper-exchanged exchange beta zeolite catalyst composition.

EXAMPLE 2 Evaluation of Back Pressures for Coated Soot Filters

The pressure drop across the uncoated and coated filters was evaluatedusing the commercially available automated equipment, Super Flow SF1020, (Probench). This equipment is designed specifically for measuringpressure drops as a function of air flow. Data from this equipmentprovides a plot of the pressure drop at ambient conditions as a functionof the air flow. The pressure drop is a measure of how easily air flowsthrough the filter. In diesel engine applications, lower pressure dropsare desirable since the engine must expend power to move the air.Therefore, the larger the pressure drop, the greater the amount ofengine power that is lost to pumping air. This lost power reduces theengine power that is available to the wheels.

FIG. 6 summarizes the pressure drop across the coated filters, CatalystA1, A2 and B1, as well as an uncoated filter of the identicaldimensions. The filters coated according to Steps (1) through (5) ofExample 1, i.e., Catalyst A1 and A2, showed a pressure drop that wasabout 25% higher than the uncoated filter. In contrast to Catalysts A1and A2, the non-optimized filter, Catalyst B1, showed pressure dropsthat were greater than 100% higher than the uncoated filter aftercoating. The pressure drop exhibited by Catalyst B1 was so high thatengine testing of this filter proved impossible. Although it is possibleto achieve lower pressure drops across filter coated in the same manneras Catalyst B1 by reducing the catalyst loading, lower levels of SCRcatalyst loading lead to unacceptable NOx reduction levels.

EXAMPLE 3 Demonstration of Particulate Removal by the SCR Catalyst

When applied to the wall flow filter, the catalyst composition shouldideally aid in the regeneration of the filter. Therefore, the SCRcatalyst composition disposed on the filter is preferably able tocatalyze the oxidation of soot and VOF portions of the particulate. Tobe effective in reducing NOx and particulate mass the SCR catalystshould preferably not oxidize ammonia or SO₂ to make SO₃. One way ofevaluating a catalyst's ability to oxidize carbon and VOF is by the useof combined Thermal Gravimetric Analysis (TGA) and Differential ThermalAnalysis (DTA). The TGA measures the weight loss of a sample while theDTA measures the change in sample's heat capacity versus a reference. Inthis experiment, a dried and calcined portion of the catalyst slurry wasmixed with 6% by weight lube oil, to simulate the VOF portion of thelube, and 14% by weight carbon black, to simulate the soot fraction ofthe particulate. The mixture was loaded into an instrument that conductsa combined TGA and DTA. Although different gas compositions can bepassed across the sample, these tests were conducted in air. The systemwas heated at a known rate to determine the weight loss and heatevolution as a function of temperature. An advantage of the technique isits ability to separate the weight loss of various soot components, andrelate these weight losses to thermal changes. Catalysts effective inburning soot will lower the onset temperature of the soot burning.

FIG. 5 plots the DTA signal in microvolts as a function of temperaturefor two catalyst compositions; (1) a reference composition, TiO₂-10 wt.% WO₃-2 wt. % V₂O₅ catalyst, and (2) the catalyst composition used tocoat Catalyst A1. The TiO₂-based composition is typical of the currentstate of art in SCR catalysts and has wide application. Powders of driedand calcined slurry of each catalyst were mixed with 6% lube oil and 14%by weight carbon black. These samples were heated at a rate of 20 C perminute, in air, from room temperature to 800° C. The resulting DTAsignal shows two peaks, one at temperatures below 400° C. correspondingto the burning of the VOF, and the second peak at higher temperaturescorresponding to the combustion of carbon black. Results show that bothcatalyst compositions were effective in burning the lube oil portion ofthe simulated particulate, but the preferred catalyst composition weremuch more effective in burning the carbon portion as evidenced by thelowering of the soot combustion temperature. As will be seen in laterexamples, this advantage is maintained without compromising the NOxreduction activity.

EXAMPLE 4 Evaluation of NOx Conversion and Particulate Removal forCoated Soot Filters

The filtration efficiency and simultaneous NOx reduction was determinedusing a prototype V6.4L turbocharged after-cooled diesel engine that isrepresentative of the current state of the art in diesel technology. Theengine was mounted on a test stand operated at steady state to providereproducible and stable emissions. The engine speed and load werecontrolled to provide a filter inlet temperature of 370° C. and a NOxconcentration of about 950 ppm. Particulate measurements were determinedaccording to the procedures described in the Code of FederalRegulations, Title 40, Part 86, paragraph 1312-88, but instead of a fulldilution tunnel, a mini-dilution tunnel was used. The dilution ratio wasdetermined from the CO₂ concentration. NOx removal on the diesel enginewas achieved by injecting a urea solution after the oxidation catalystand before the SCR coated filter substrate. The experimental arrangementis illustrated in FIG. 7. NOx and ammonia were measured using a FTIRinstrument equipped with a heated sampling line and analysis cell. NOx,CO and HC were also determined using a Horiba analysis bench, designedspecially for the analysis of raw diesel exhaust.

Additional catalysts were prepared and aged for 1000 hours on astationary diesel engine using an aging cycle that simulated passengercar driving. The aging cycle was an adaptation of the proceduredescribed in “Durability Driving Schedule for Light Duty Vehicles andLight Duty Trucks” Code of Federal Regulations, Part 86 paragraph836-01. The test cycle described therein specifies speeds and periodicstops for a vehicle driven around a test track. From previous work, thetemperature profile of this cycle was measured, and then simulated on anengine bench. The aging and the evaluation used ARCO ECD diesel fuel.This fuel has a sulfur content of 12 ppm, consistent with the fuelexpected to be available during the expected application of thetechnology.

Using the experimental configuration shown in FIG. 7, the NOx conversionand particulate removal were determined for three catalyst substrates.As seen in FIG. 7, the experimental configuration included a ureainjector, upstream of the catalyzed soot filter, and an oxidationcatalyst (DOC) upstream of the urea injector. To eliminate any of avariation due to the DOC, all of the trials were conducted with the sameDOC. The oxidation catalyst composition was disposed on a 5.66×6 in flowthrough cordierite substrate. The oxidation catalyst compositioncontained 90 g/ft³ dispersed on γ-alumina, and contained 27 wt. %hydrogen ion exchanged-beta zeolite. The DOC was aged 1000 hours.

In the trials conducted in this experiment, the SCR catalyst compositionwas disposed on either a wall flow monolith substrate or a flow throughmonolith substrate. The SCR catalyst composition was identical to thatused to coat the substrates in Example 1, i.e., it contained acopper-exchanged zeolite with a zirconia binder. In particular, thesubstrates used in the experiment were: a fresh catalyst substrateprepared identically to Catalyst A1 in Example 1 (designated as CatalystA1 _(fresh)); a separate catalyst substrate also coated identically toCatalyst A1, but aged 1000 hours (designated as Catalyst A1 _(aged));and finally the third catalyst substrate which was of the flow throughtype, prepared identically to Catalyst D1 (designated as Catalyst D1_(fresh)).

Table 1 below summarizes particulate filtration efficiency and NOxreduction for the three catalyst substrates. The filtration efficiencywas determined with and without urea injection.

TABLE 1 NH₃ Total Trial Substrate % NOx slip, Particulate # CatalystType NH₃/NOx conv. ppm Removal, % 1 D1_(fresh) flow through 0 <5 0 <10 2D1_(fresh) flow through 0.3 30 0 <10 3 A1_(fresh) wall flow 0 <5 0 82 4A1_(fresh) wall flow 0.5 51 0 85 5 A1_(aged) wall flow 0 <5 0 81 6A1_(aged) wall flow 0.5 55 0 85

As can be seen in Table 1, disposing the SCR catalyst composition on thewall flow monolith, did not cause a loss of NOx removal efficiency.Moreover, the filtration efficiency is unaffected by urea injection.While the SCR coated flow monolith provided NOx removal function, itlacked high filtration efficiency demonstrated by the coated, wall flowmonoliths. Thus, these coated SCR filter substrates of the inventiondemonstrate integrated, high NOx and particulate removal efficiency.

Moreover, the durability of the SCR catalyst composition is demonstratedby the data in Table 1. Aging the coated substrate caused neither a lossof filtration efficiency nor NOx removal efficiency.

While this invention has been described with an emphasis upon preferredembodiments, it will be obvious to those of ordinary skill in the artthat variations in the preferred devices and methods may be used andthat it is intended that the invention may be practiced otherwise thanas specifically described herein. Accordingly, this invention includesall modifications encompassed within the spirit and scope of theinvention as defined by the claims that follow.

1. A catalyst article comprising a wall flow monolith having a plurality of longitudinally extending passages formed by longitudinally extending walls bounding and defining said passages, wherein the passages comprise inlet passages having an open inlet end and a closed outlet end, and outlet passages having a closed inlet end and an open outlet end, and the wall flow monolith contains an SCR catalyst composition at a loading that achieves suitable NOx conversion efficiency but does not cause an unacceptable increase in backpressure when used in a diesel exhaust system.
 2. The catalyst article of claim 1, wherein the SCR catalyst composition is in the form of a washcoat on the walls of the monolith.
 3. The catalyst article or claim 2, wherein the SCR catalyst composition permeates the walls of the monolith at a concentration of at least 1.3 g/in³.
 4. The catalyst article of claim 3, wherein there is from 1.6 to 2.4 g/in³ of SCR catalyst composition disposed on the wall flow monolith.
 5. The catalyst article of claim 1, wherein the SCR catalyst composition is effective to catalyze the reduction of NOx at a temperature below about 600° C. and able to aid in regeneration of the filter by lowering the temperature at which soot captured by the filter is combusted.
 6. The catalyst article of claim 1, wherein the SCR catalyst composition has a thermal resistance to degradation at temperatures greater than 650° C.
 7. The catalyst article of claim 6, wherein the SCR catalyst composition is effective to resist degradation upon exposure to sulfur components.
 8. The catalyst article of claim 1, wherein the SCR catalyst composition promotes the oxidation of excess NH₃ with O₂.
 9. The catalyst article of claim 1, wherein the SCR catalyst composition comprises a zeolite and base metal component selected from one or more of a copper and iron component.
 10. The catalyst article of claim 9, wherein the base metal component is a copper component.
 11. The catalyst article of claim 10, wherein the zeolite of the SCR catalyst composition has a silica to alumina ratio of at least about
 10. 12. The article of claim 11, wherein the zeolite of the SCR catalyst composition is selected from beta zeolite, USY and ZSM-20.
 13. The catalyst article of claim 11, wherein the zeolite of the SCR catalyst composition is a beta zeolite.
 14. The catalyst article of claim 9, wherein the zeolite of the SCR catalyst composition has pores with a pore diameter of at least about 7 Angstroms and are connected in three dimensions.
 15. The catalyst article of claim 9, wherein the SCR catalyst composition contains one or both of an iron and a copper promoter present in an amount of from about 0.1 to 30 percent by weight of the total weight of promoter plus zeolite.
 16. The catalyst article of claim 9, wherein the SCR catalyst composition contains one or both of an iron and a copper promoter present in an amount of from about 1 to 5 percent by weight of the total weight of promoter plus zeolite.
 17. The catalyst article of claim 3, wherein the wall flow monolith has a wall porosity of at least 50% with an average pore size of at least 5 microns.
 18. The catalyst article of claim 3, wherein the wall flow monolith has a wall porosity of from 50 to 75% with an average pore size of from 5 to 30 microns.
 19. A catalyst article comprising a wall flow monolith having a plurality of longitudinally extending passages formed by longitudinally extending walls bounding and defining said passages, wherein the passages comprise inlet passages having an open inlet end and a closed outlet end, and outlet passages having a closed inlet end and an open outlet end, wherein the wall flow monolith comprises an SCR catalyst composition that permeates the walls at a concentration of at least 1.3 g/in³; wherein the wall flow monolith has a wall porosity of at least 50% with an average pore size of at least 5 microns.
 20. The catalyst article of claim 19, wherein there is from 1.6 to 2.4 g/in³ of SCR catalyst composition disposed on the wall flow monolith. 